Method of making a dispersion-hardened ferrous alloy

ABSTRACT

1. A PROCESS FOR PRODUCING DISPERSION-HARDENED FERROUS ALLOYS COMPRISING MAKING A MIXTURE OF FERROUS METAL AND ALUMINUM OXIDE, RANDOMLY DISTRIBUTING SAID ALUMINUM OXIDE THROUGHOUT SAID FERROUS METAL BY PASSING INDUCED CURRENT SUPPLIED BY A HIGH-FREQUENCY POWER SOURCE THROUGH THE MIXTURE TO MELT THE METAL AND STIR THE MIXTURE AND COOLING THE MIXTURE TO PRODUCE A SOLIDIFIED BODY CONTAINING A FINE DISPERISION OF ALUMINUM OXIDE.

United States Patent 3,667,932 METHOD OF MAKING A DISPERSION- HARDENED FERROUS ALLOY Horace Pops, Edgewood Borough, Pa., assignor to United States Steel Corporation No Drawing. Filed Aug. 20, 1964, Ser. No. 390,992 The portion of the term of the patent subsequent to Dec. 6, 1987, has been disclaimed Int. Cl. (121d 1/00 US. Cl. 75--.5 BC 9 Claims This invention relates tothe production of dispersionhardened alloys and in particular to dispersion-hardened ferrous alloys. The terms ferrous, ferrous metal, and ferrous alloy, as used herein refer to iron and iron alloys.

Dispersion hardening has proven to be a useful method of improving the physical and mechanical properties of metals and alloys. Dispersion hardening generally involves the introduction of a finely dispersed second phase into a metal matrix. The improved properties that result from dispersion-hardening are a direct result of the particles functioning as a barrier or obstacle to dislocation motion in the matrix. In addition, the dispersoid as used herein refers to a dispersed phase in a multi-phase system.

A typical advantage of dispersion hardening, as a corollary to the increased resistance to dislocation motion, is the improvement obtained in creep resistance. Steels can be improved by dispersion hardening in the same way the precipitation hardening improves their properties. In manganese steels, dispersion hardening can improve the yield strength-in stainless steels it can improve the fatigue strength.

Prior to this invention metals were dispersion hardened primarily by powder metallurgical techniques involving sintering. In this way it was possible to obtain some degree of control of distribution of the dispersoid in the metal matrix.

Melting of the metal base has been avoided because melting causes segregation of the typically lighter dispersoids from the heavier metal matrix material. Powder metallurgical methods involve mixtures of solid materials and it is very difficult to obtain prefectly random mixtures because the finer dispersoid particles frequently adhere together. Moreover, ideal blending of powders is difficult to accomplish. Such techniques are also limited to small scale operations.

The present invention avoids the difficulties of prior practices and provides a method useful for large-scale production using conventional melting and casting techniques. A dispersion-hardened alloy produced by practicing the method of the invention contains the hardener randomly distributed throughout the metal matrix and is especially suitable for high temperature applications.

According to the invention, a dispersion-hardened alloy is made by preparing a mixture of a metal and a hardener. The hardener is distributed throughout the metal by the turbulence of a thermite reaction or by passing induced current supplied by a high-frequency power source through the mixture to melt the metal and stir the mixture. The mixture is then cooled to produce a solidified body containing a line dispersion of hardener within the metal matrix.

Dispersion-hardened alloys can be made according to the invention by several embodiments. In one embodiment the hardener is dispersed within a metal matrix by being formed in situ in a thermite-type reaction. It is also possible to produce dispersion-hardened alloys according to the invention by starting with mixtures of hardener and some electrically conductive metal which will form the matrix. In the latter embodiment the hardener is randomly distributed through the metal base by passing an induced current from a high-frequency power 3,667,932 Patented June 6, 1972 source through the mixture. The induced current melts the metal and stirs the mixture to distribute the hardener. It is also possible, in practicing the invention, to combine the thermlte-type reaction with mixing 'by indu ed current flow to produce an improved dispersion-hardened alloy.

One advantage of the present invention compared to powdered metallurgical techniques is that the particle size of the hardener does not depend upon the particle size of the initial Phases. In my process, the hardener is introduced into and dispersed throughout a molten metal mabefore casting or solidifying. In conventional melting and casting processes, nonwettable particles cannot be forced beneath the melt surface. Accordingly, good liquidsolid dispersions cannot be formed because successful mix- 1ng requires that the liquid wet the solid particles. This disadvantage has been avoided by the invention because the particles of hardener are either produced by a therrnite reaction within the melt or are stirred into the molten metal by induced current flow.

The hardener is produced in the melt in situ by a thermite reaction and distribution of the particles produced in the melt is accomplished by the violent turbulence accompanying the usually, highly exothermic, thermite reactions. The turbulence is also responsible for the formation of extremely small particles which result in a more satisfactory product and for minimizing the tendency for the particles to agglomerate.

In lieu of in situ formation of hardener, the dispersion can be accomplished by using induced current to melt the metal component and agitate the liquid-solid mixture to distribute the solid particles of hardener. For this purpose, induced current from a high-frequency power source is necessary to achieve the agitation acquired for thorough mixing. The induced current flow can be generated either in an induction furnace or by the relatively new levitation melting techniques such as are disclosed in US. Pats. 2,664,496 and 2,686,864. Induction furnaces employ a shell of nonconducting material and use high-frequency power applied to a primar coil. High frequency power applied to the primary coil creates a magnetic flux which passes through the charge which is disposed in the center of the coil. The charge acts as the secondary winding of a single turn transformer and the induced current melts the charge by heat developed due to the electrical resistance of the charge. The magnetic flux and the induced current provide sufficient agitation of the liquid-solid mixture to enable the mixture to be cast and cooled into solidified bodies in which the solid particles are randomly. distributed throughout the metal matrix.

Levitation melting also employs induction current from an electromagnetic force created by a high-frequency power source. By this technique, alternating magnetic fields are employed which not only produce melting and levitation of the electrically conductive material, but strong agitation of the mixture as well. Vigorous mixing resulting from the magnetic fields promotes random dispersion of the solid particles in the liquid. Levitation melting furnaces employ a conical coil connected to generator leads. At the lower portion of the coil the turns are formed in one direction and at the top portion the turns are oppositely directed. Current in the lower portion produces an electromagnetic field which is opposed by the field in the upper turns, resulting in a weak field in the center of the coil. When an electrically conductive material is placed in the center, it is pushed toward the weak field and can be levitated. The induced current can heat the electrically conductive material to its melting point rapidly and cause effective stirring of the mixture.

Although the process can be used generally to produce dispersion hardened alloys of electrically conductive metals, it is particularly suitable for making dispersion hardened ferrous alloys. One ferrous'alloy which has been discovered to be particularly outstanding is a combination of ferrous metal and aluminum oxide. Particles of aluminum oxide can be produced in situ in a ferrous metal matrix by a thermite reaction. This reaction is represented by the following unbalanced chemical equation:

(Fe-Al) Liquid-l-(FeO) Solid (-Fe) Liquid+ (A1 Solid Since aluminum oxide is more stable than iron oxide, the

aluminum reacts with the FeO in an exothermic reaction. This thermite reaction not only produces aluminum-oxide particles within the melt but also creates sufficient turbu lence that the oxide particles are randomly dispersed in the liquid metal. As an illustration of this embodiment, 1.53 grams of high purity FeO powder, 48.07 grams of -325 mesh Fe powder, and 0.4 gram of .Al in the form of crushed ferro-aluminum (F e-Al) were mixed and compacted by cold pressing. The compact was placed in a levitation-melting apparatus, then melted and cast into copper molds under a helium atmosphere. The resulting alloy contained oxide particles approximately 0.7 micron in diameter which were 18 microns apart and had a volume fraction of 5%. No segregation or agglomeration of oxide particles at the top or bottom of the ingot was observed.

It is also possible to produce a dispersion hardened alloy by difiusing oxygen gas into a melt of the matrix metal. This reaction involved an exothermic interaction between molten ferro-aluminum alloy and oxygen as follows:

(Fe-Al) Liquid+ (O GaseFe (Liquid) +Al O (Solid) The oxygen is diffused into the liquid and reacts with the aluminum to form aluminum oxide. As an example of this embodiment, 0.4 gram of Al and 49.6 grams of Fe powder were compacted and then melted under a 75% oxygen-25% argon atmosphere. An exothermic reaction was observed that was associated with the diffusion of oxygen into the liquid, the reaction of the gas with the aluminum and the subsequent formation of aluminum oxide. A product containing very fine, oxide particles randomly dispersed in a ferrous matrix was obtained.

\As discussed above, ferrous-aluminum oxide alloys can be produced from mixtures of ferrous metal and aluminum oxide. In this case, the successful production of the alloy is dependent upon the efficient mixing resulting from an induced current flow by a high frequency power source. This embodiment is illustrated by an example in which 49.5 grams of Fe and 0.5 gram of powdered A1 0 were mixed and compacted by cold pressing. The compact was levitation melted under a helium atmosphere and then cast into a copper mold. The resulting alloys contained fine, i.e. less than one micron in diameter, spheroidal dispersoids, uniformly distributed in the matrix.

The combination of the thermite reaction and mixing by induced current cflow can also be employed to produce superior dispersion hardened alloys. -In one example, 447 grams of type 304 steel were melted in a magnesia crucible by induction heating under an argon atmosphere. First, 1.4 grams of aluminum were added and then 5.2 grams of iron oxide powder were added. Small spheroidal aluminum-oxide particles were formed by a thermite reaction between the aluminum and iron oxide within the melt. Ingots were cast by a rapid solidification of the melt in a preheated cast-iron mold. Very fine oxide particles were found randomly distributed throughout the ferrous metal matrix.

Thermite reactions used in the invention are based upon the thermodynamic principle that the degree of completion of a given reaction is represented by the standard free energy of formation increases numerically, the reaction becomes more favorable. The thermite reaction is 4 relatively turbulant and produces large heat evolution with the result that the random dispersion of oxide particles produced in the melt can be maintained when the matrix is solidified by cooling in a mold of suitable material, e.g. copper, graphite, etc.

In addition, thermite reactions result in the formation of very fine particles and I have found that ferrousaluminum oxide alloys can be produced in which the aluminum oxide particles are less than about 1 micron in size. Such small particles are necessary to satisfactorily hinder dislocation motion in the alloy.

Melting can be'performed if desired under inert or controlled atmospheres. 'In some cases, helium or argon atmospheres can be used to minimize contamination of the product. The invention is applicable to ferrous base alloys and can be used to form hardeners such as zirconium oxide, silicon oxide, etc., by techniques as described above. In such cases, the elements zirconium, silicon, titanium, thorium, etc., whose oxides are more stable, i.e. have a numerically larger negative free energy than the oxide material, e.g. iron oxide, will'react with the iron oxide or oxygen to form their oxides. The amount of hardener which can be included in the metal alloy can vary within wide limits and alloys with as much as 15 volume percent of the hardener can be made.

I claim:

1. -A process for producing dispersion-hardened ferrous alloys comprising making a mixture of ferrous metal and aluminum oxide, randomly distributing said aluminum oxide throughout said ferrous metal by passing induced current supplied by a high-frequency power source through the mixture to melt the metal and stir the mixture and cooling the mixture to produce a solidified body containing a fine dispersion of aluminum oxide.

2. A metal according to claim 1 wherein said mixture of ferrous alloy and aluminum oxide is made by a thermite reaction of aluminum and iron oxide.

'3. A method according to claim 1 wherein said mixture of ferrous alloy and aluminum oxide is made by a reaction of ferro-aluminum and oxygen.

4. A method according to claim 1 wherein said mixture of ferrous alloy and aluminum oxide is made by physically mixing particles of ferrous metal and aluminum oxide to provide an admixture of the two.

5. A method of making a dispersion-hardened ferrous alloy comprising forming a compressed, coherent mass containing ferrous metal and aluminum oxide, melting said ferrous metal and randomly distributing said aluminum oxide throughout said ferrous metal by passing induced current from a high-frequency power source through said mass, and cooling .to produce a solidified dispersion-hardened ferrous alloy containinga random distribution of aluminum oxide particles.

6. A method of making a dispersion-hardened ferrous alloy comprising preparing a mixture of ferrous metal, iron oxide, and at least one other metal Whose oxide has a negative standard free energy numerically larger than iron oxide, reacting said other metal with iron oxide to form oxide particles of said other metal, generating sufiicient turbulence in the reaction mixture to randomly distribute said oxide particles throughout said ferrous metal and thereafter cooling to produce a solidified ferrous alloy dispersion-hardened by the presence of said oxide as fine particles distributed throughout a matrix of said ferrous metal.

7. A method of making a dispersion-hardened ferrous alloy comprising preparing a mixture of ferrous metal, iron oxide, and aluminum, reacting said iron oxide and aluminum to form aluminum oxide and to create sufiicient turbulence in the reaction mixture to randomly distribute said aluminum oxide throughout said ferrous metal and cooling to produce a solidified ferrous alloy containing particles of aluminum oxide less than about out a ferrous metal matrix.

8. A method of making a dispersion-hardened ferrous alloy comprising preparing a mixture of ferrous metal, iron oxide and aluminum in a coherent mass, melting said ferrous metal by passing through it induced current from a high-frequency power source and reacting said iron oxide and aluminum to produce fine particles of aluminum oxide, said induced current randomly distributing said ferrous metal, and thereafter cooling to produce a solidified ferrous-aluminum alloy wherein thealuminum oxide is present as a random distribution of fine particles in a 10 with aluminum in the melt to form particles of aluminum 15 oxide, and cooling said melt to produce a solidified ferrous alloy containing particles of aluminum oxide distributed throughout a ferrous matrix.

References Cited UNITED STATES PATENTS 2,642,546 6/1953 Ahrens 29-192 R 2,686,864 8/1954 Wroughton et a1. 75-10 R 2,957,232 10/1960 Bartlett 29-192 R FOREIGN PATENTS 1,810 1900 Great Britain 75-12 1,255,349 1/1961 France 75-12 ALLEN B. CURTIS, Primary Examiner US. Cl. X.R.

writer STATES rlirmr @FFICE @ER'llllfii'iE @l QQRREUHQN Patent No. 3,667,932 Dated June Q, 1.9?2

Inventor HOlfiCQ Popg It is certified that error appears in the above-identified patent and that said Letters Patentare hereby corrected as shown below:

Column 1, line 23, after "the", second occurrence,

insert diepersoids tend to create a fi ne--,-grained structure. The term "-5 column 3, line 71;, after "formation", insert M a That is, as the negative standard free energy of formation ---5 column 4, line 35, "metal" should read method column 5, line 8, before "ferrous", insert aluminum oxide particles through said in the first reference "2,642,5 i6" should read 2,6 2,65 4- Signed and sealed this 19th day of December i972.

(SEAL) Atteet:

EDWARD MELETCEERJR, ROBERT GOTTSCHALK attesting Officer Gommissioner of Patents FORM PO-1OS0 (10-69) USCOMM-DC 60376-P69 U5. GOVERNMENT PRINTING OFFICE I969 0-866-334.

Patent No. 3,667,932 Dated June 6 1972 Invent0r(S) HOrace PS It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 23, after "the", second occurrence,

insert dispersoids tend to create a fine-grained structure. The term ----5 column 3, line 7).;., after f ormation", insert a That is, as the negative standard free energy of formation column 4, line 35, "me't'el' should read I method column 5, line 8, before "ferrous", insert aluminum oxide particles through said --3 in the first reference "2,642,546" should read 2,642,654

Signed and sealed this 19th day of December 1972.

(SEAL) Attest:

EDWARD MELETCI-ERJR. ROBERT GOTTSCHAIK Attesting Officer 7 Commissioner of Patents ORM PO-1050 (10-69) USCOMM-DC GOING-P69 u.s. GOVERNMENT PRINTING OFFICE: I969 o-asa-aam 

1. A PROCESS FOR PRODUCING DISPERSION-HARDENED FERROUS ALLOYS COMPRISING MAKING A MIXTURE OF FERROUS METAL AND ALUMINUM OXIDE, RANDOMLY DISTRIBUTING SAID ALUMINUM OXIDE THROUGHOUT SAID FERROUS METAL BY PASSING INDUCED CURRENT SUPPLIED BY A HIGH-FREQUENCY POWER SOURCE THROUGH THE MIXTURE TO MELT THE METAL AND STIR THE MIXTURE AND COOLING THE MIXTURE TO PRODUCE A SOLIDIFIED BODY CONTAINING A FINE DISPERISION OF ALUMINUM OXIDE. 